Power systems such as electrical power distribution or transmission systems generally include a protection system for protecting, monitoring and controlling the operation and/or functionality of other components included in the power system. Such a protection system may for example detect short-circuits, over-currents and over-voltages in power lines, transformers and/or other parts or components of the power system. The protection system can include protection equipment such as circuit breakers for isolating any possible faults for example occurring in power transmission and distribution lines by opening or tripping the circuit breakers. After the fault has been cleared, e.g. by performing repairs and/or maintenance on the component in which the fault has been detected, the power flow can be restored by closing the circuit breakers. Alternatively or optionally, the protection system can be arranged to, upon detection of a fault in a particular route for power flow, isolate the route in which the fault has been detected and select an alternative route for the power flow.
Differential protection is a method for protection of power systems that is based on comparing currents on sides of a protected zone or a protected unit. A protected unit could in principle include or be any part or component of the power system, for example a transmission line, transformer, generator, and/or a transformer station busbar. Differential protection is a widely used type of protection in power systems due to advantages such as speed, reliability, sensitivity and selectivity. The differential protection principle is based on Kirchhoff's current law, and is usually implemented by summation of all the measured currents entering and leaving the protected unit and detecting a fault if the sum deviates from zero, hence indicating that some abnormal current path exist, i.e. a fault, in the protected unit. However, since ‘regular’ circuit theory no longer holds when the propagation time in a circuit is not negligible, performance of the differential protection may be degraded in such cases. For example, when the differential protection is applied in protected units where the power flow or current path is relatively long, e.g. where the protected unit is a relatively long transmission line such as an overhead transmission line (OHL) or a direct current (DC) cable, fault detection reliability and/or accuracy may be degraded due to time of propagation and reflections of travelling waves in the protected unit. By a “relatively long” current path or transmission line it is generally meant a current path or transmission line that cannot be accurately represented by a ‘lumped’ transmission line model or lumped element model, i.e. a current path or transmission line for which the transmission line parameters or attributes such as impedance, speed of wave propagation and bandwidth of the waveform of a wave are distributed throughout the material of the current path or transmission line. This is generally the case where the current path or transmission line has a length that is relatively large compared to the wavelength or a fraction of the wavelength of the operating frequency of the current path or transmission line. In non steady state, time of propagation and reflections of travelling waves in the protected unit can cause the currents in the line ends to differ, in particular during transients caused by e.g. external faults, and thereby give rise to a detection of a fault in the protected unit even though there is no fault at the time. In turn, this can cause an increase in unwarranted trips of circuit breakers in the power system, thereby causing an unwarranted interruption in power flow.